1. Technical Field
This invention relates generally to a navigation system, and more particularly to the generation of standard heights with a satellite navigation system.
2. Background of the Invention
The height generally depicted on topographic maps and physical markers is the standard height H. The standard height H indicates the height above mean sea level, and is determined by geodetic leveling. The standard height system may sometimes be referred to as the orthometric height system or the normal height system.
The National Geodetic Survey determines heights for the standard height system in the United States by taking physical measurements and using geodetic leveling. Government agencies in other countries perform similar functions, such as the State Survey Authority of the Federal State of Baden Württemberg (Landesvermessungsamt Baden Württemberg) in Germany, and the Federal Office of Topography (Bundesamt für Landestopographie) in Switzerland.
Heights obtained from satellite navigation systems, such as the Global Positioning Systems (GPS) of the United States or the Global Orbiting Navigation Satellite System (Glonass) of the Russian Federation, utilize a different height system than those obtained with geodetic leveling. Satellite navigation system data is generally processed to obtain an ellipsoidal height h. An ellipsoidal height h is a height above or below a simple ellipsoid model of the Earth, such as the World Geodetic System 1984 (WGS84) ellipsoid model of the Earth.
Because different height systems are used, the ellipsoidal height h indicated by a satellite navigation system for a location may differ by as much as 50 meters from the standard height H indicated on a printed map for the same location. This difference may present a problem when trying to reconcile the height provided by a printed map with the height provided by a satellite navigation system. Even though many vehicle navigation systems included digital maps, this difference in heights may be a problem when traveling either off-road or in an area outside of the digitally mapped area.
To convert from an ellipsoidal height h to a standard height H may require an undulation value N. The relationship between the ellipsoidal and standard height systems may be expressed by the equation: h=H+N. Accurate transformation between ellipsoidal height h and standard height H requires a high resolution undulation N model of comparable accuracy to the measured height data. Two examples of such models are the GEOID96 model in the United States and the Digitale Finite Element Höhenbezugsfläche (DFHBF) model in Germany.
It would be desirable to provide a satellite navigation system for use in a vehicle that indicates a standard height H for a location. It would be further desirable to provide a satellite navigation system for use in a vehicle that provides a standard height H for a section of a road. Additionally, it would be desirable to provide a satellite navigation system for use in a vehicle that provides a standard height H for a section of a road when the vehicle is on a road, and also provides standard height H for an off-road location when the vehicle is not on a road.